The following application relates to a thin film electroluminescent device.
Thin films of rare earth doped alkaline earth sulfides such as cerium doped strontium sulfide have been extensively investigated for applications in full color alternating current thin film electroluminescent (ACTFEL) display devices. Such a device is shown in Barrow et al., U.S. Pat. No. 4,751,427, incorporated by reference herein. The emission spectrum of SrS:Ce is very broad covering both blue and green portions of the visible spectrum, i.e., 440 to 660 nm with a peak at around 500 nm. A full color ACTFEL display device can be obtained by adding a red emitting phosphor, for example CaS:Eu or one that has a red component in its emission spectrum. With such a combination of films, one can build a white light emitting phosphor stack. White phosphor structures can then be laminated with primary color filters to build a color display which is very cost effective in terms of production.
With white light emitting phosphor stacks, however, the blue portion of the emission spectrum can be rather weak, particularly strontium sulfide phosphor doped with cerium which in the past has been the most promising of the blue emitting phosphors. Only about 10% of the original luminance can be obtained after filtering if a nearly blue color is to be achieved. For blue coloration in the CIE range of x=0.10, y=0.13 the transmission ratio is further reduced to only about 4%. Therefore, to produce a color display with acceptable luminance, it is necessary to use a lighter blue color filter but this in turn leads to a compromised blue chromaticity. Any display fabricated with such a poor blue chromaticity has a limited color gamut and is unable to produce the range of colors available with CRT or LCD technology.
Therefore, in order to achieve a high performance color ACTFEL display, the blue emission efficiency of the EL phosphor thin film must be greatly improved. In U.S. Pat. No. 4,725,344, Yocom, et al., a method is disclosed for forming alkaline earth sulfide luminescent films by chemical reaction between alkaline earth metal halide and hydrogen sulfide on heated substrates. Yocom, et al. does show a strontium sulfide thin film phosphor which has a more bluish color (CIE x=0.17, y=0.25) than an unfiltered SrS:Ce device. However, the luminance performance of the Yocom et al. device is not high enough for practical application. Experimentation has also been reported regarding SrS:Cu devices which are prepared by sputtering, for example in Ohnishi et al., proceedings of the SID 31/1, 31 (1992). The Ohnishi et al. device, however, is even dimmer than the Yocom et al. device (and no color data is available).
Lehmann, in a paper titled xe2x80x9cAlkaline Earth Sulfide Phosphorous Activated by Copper, Sulfur, and Gold,xe2x80x9d reported that strontium sulfides doped with mono-valent ions with D10 configuration, e.g., Cu+,Ag+ plus, emit green and blue light, respectively, when excited by an electron bombardment. Lehmann was attempting to develop a powder phosphor material suitable for cathode ray tube devices and thus are considered unsuitable for alternating current (AC) and thin film electroluminescent devices.
The first blue emitting SrS:Cu electroluminescent device suitable for alternating current, thin-film electroluminescent devices was reported by Kane et al., in a paper entitled xe2x80x9cNew Electroluminescent Phosphorous Based on Strontium Sulfide.xe2x80x9d However, the device performance was very poor, e.g., less than 1.0 CD/M2 at 60 Hertz.
Sun et al., in a paper entitled xe2x80x9cA Bright And Efficient New Blue TFEL Phosphor,xe2x80x9d Proceedings of 17th IDRC, Toronto, Canada, p.201 (1997), disclosed a new phosphor, namely, SrS:Cu+, with a luminous performance that was increased over the prior known blue phosphorous in a range of 30. After further development, Sun, in a paper entitled xe2x80x9cBlue Emitting SrS:Cu TFEL Phosphor Developmentxe2x80x9d, Conference record of the 1998 International Display Research Conference, Asia Display, 1998, Seoul, Korea CD-ROM (1998), disclosed an improved blue emitting phosphor material of SrS:Cu,Ag having twice the luminance efficiency of SrS:Ce due to the enhanced blue emission at wavelengths below 460 nm.
Troppenz., et al., in a paper entitled xe2x80x9cElectroluminescence of SrS:Cu and SrS:Cu,Ag at High Ambient Temperaturesxe2x80x9d shows that SrS:Cu and SrS:Cu,Ag have severe thermal quenching properties. Thermal quenching refers to a reduction in luminance and a concomitant reduction in transferred charge, when an alternating current thin-film electroluminescent device is operated at an elevated temperature. Typically, thermal quenching refers to a reduction in the luminescence of a phosphor when it is operated at an elevated temperature. Normally, thermal quenching is considered within the context of a configuration coordinate diagram and is attributed to a decreasing radiative recombination efficiency of the phosphor as the phonon density increases with increasing temperature. Thus, traditional thermal quenching is due exclusively to an optical effect associated with the temperature dependence of the radiative recombination efficiency.
In contrast, electroluminescence thermal quenching may be employed to denote a reduction in luminescence that is in excess of normal thermal quenching and is associated with a concomitant reduction in transferred charge. Thus, electroluminescent thermal quenching arises from a thermally-activated electrical effect in addition to the optical effect associated with normal thermal quenching.
Referring to FIG. 1, the luminance of a SrS:Cu device is shown at 25xc2x0 C. and at 50xc2x0 C. It may be observed that the increase in temperature resulted in nearly at 57% luminance loss at the increased temperature. A similar decrease in luminance likewise occurs with SrS:Cu,Ag phosphor materials. This level of thermal quenching for SrS:Cu and SrS:Cu,Ag is simply too high for a dependable display device.
The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.